1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly, to a semiconductor laser with a inner stripe structure.
2. Description of the Related Art
Semiconductor lasers, compared with gas or other solid state lasers, have many advantages such as compactness, high efficiency, high speed response, and high reliability.
Therefore, semiconductor lasers are used more and more as light sources especially for optical disk memory systems and optical communication systems. Optical disk memory systems require shorter wavelengths than optical communication systems. AlGaAs type lasers satisfy this requirement and so many of them are used in massproduced systems such as compact disk (CD) players. However, it is necessary to increase their output power when rewritable optical disk memory systems are developed.
A rewritable optical disk is a read and write memory, while a CD is a read only memory. Information is optically written, read and erased in the rewritable optical disk memory systems. When erasing and writing, much more laser power is needed than when reading. Thus, there appeared a demand for higher power AlGaAs type lasers. These lasers are required to be transverse- mode-stabilized as are the lasers for CD players. This mode stabilization is achieved by suppressing higher order transverse modes.
The inner stripe structure as well as other mode-stabilizing structures stabilizes the transverse mode by restricting laser beam width narrow enough to cut off higher order modes. Most CD players use the lasers with this structure because it is much easier to make than other structures. However, the output power of these lasers is limited to below about 10 mW, while rewritable optical disk memory systems require 30 mW at least. Output power of semiconductor lasers is limited by chatastrophic optical power damage (COD) at laser facet in most cases. COD is caused by optical power absorption in the active layer near a facet. Therefore, the maximum output power can be increased by making the active layer effectively transparent in the region near each facet. This region is called the window or window region while the region between the two windows is called a gain region. The semiconductor lasers having such windows are called window structure lasers. The window can be achieved by making the bandgap energy of the active layer higher and/or making the optical power density lower near the facet than in the middle of the laser cavity. To achieve the windows, many structures are proposed.
Applied Physics Letter, vol. 42 No. 5, p406 (1987) and Japanese Patent Disclosure (Kokai) No. 57-211791 disclose a window structure laser with an inner stripe structure. The basic structure of this prior art is shown in FIG. 1. This structure comprises a substrate 5 which is composed of a p-type GaAs basement 7 and an n-type GaAs layer 8, which acts as a current-blocking-layer (CBL), a channel 3 etched into the CBL 8 to reach the basement 7, a p-type Al.sub.x Ga.sub.1-x As cladding layer 9, a p- or n-type Al.sub.y Ga.sub.1-y As (0&lt;y&lt;x&lt;1) active layer 4, an n-type Al.sub.x Ga.sub.1-x As cladding layer 10, an n-type GaAs ohmic contact layer 11, and a pair of cleaved facets 12A and 12B which are perpendicular to the channel 3 and act as cavity mirrors to compose a laser cavity. The laser cavity length is about 300 .mu.m. The pumping current injected into the active layer 4 is confined just above the channel because of the CBL 8. As implied by the layer names, the two cladding layers 9 and 10 and the active layer 8 make up an optical wave guide and the laser beam propagates in the active layer 4. The width of the optical wave guide is restricted by the width of the channel 3 because outside the channel 3, the cladding layer 9 is too thin to confine the laser beam in the active layer 4. In semiconductor lasers with an inner stripe structure, the laser beam width should be about 4 .mu.m or less to cut off higher order transverse modes. However, the channel width is about 7 .mu.m in the region 2 and higher order transverse modes might be easily excited. Therefore, the channel 3 is narrowed to about 4 .mu.m in the regions 1A and 1B expecting the higher order modes to be filtered out by these parts. The four layers 4, 9, 10, and 11 are grown by liquid phase epitaxy (LPE) on the substrate 5. In LPE growth on such concaved surface as the channel surface, crystal growth velocity becomes faster as the curvature of the surface is increased. This is considered to be caused by the interface energy between the crystal and the melt. Therefore, the curvature of the epitaxial layer reduces faster in the narrow channel than in the wide channel. Thus, the surface of the p-type Al.sub.x Ga.sub.1-x As cladding layer 9 can still be concave in the wide channel when it becomes flat over the narrow channel. Then, the active layer 4 is sagging and crescent-shaped in the wide channel and flat and relatively thin over the wide channel. Thus, we gain an active layer as thin as 0.05 .mu.m or less near the facets while as thick as 0.10 .mu.m or more in the middle of the laser cavity. When an active layer is much thinner than the laser light wavelength of about 0.2 .mu.m as it is in this structure, the laser beam is not perfectly confined in the active layer and enlarges its cross-section. Consequently, the optical power density is reduced as the active layer 4 becomes thinner. Therefore, the regions 1A and 1B, where the active layer 4 is thinner than in the region 2, become windows and the region 2 becomes a gain region. Moreover, the bandgap energy of the active layer 4 is higher in the regions 1A and 1B than in the region 2. This helps the regions 1A and 1B act as windows. This bandgap difference comes from the characteristics peculiar to the LPE growth that Al concentration in the Al.sub.x Ga.sub.1-x As increases as the LPE growth velocity decreases.
Thus, this prior art gives the window structure laser with a inner stripe which emits optical power over 70 mW. However, this structure shows transverse mode instability. This instability causes nonlinearity in its optical power versus driving current relation as shown in the FIG. 2 as well as asymmetric and power depending far-field-patterns as shown in FIG. 3. These poor characteristics make it impossible to use this laser in a rewritable optical disk memory system. This transverse mode instability reveals that higher order transverse modes are excited in the gain region and failed to be filtered out by the window regions. This significant drawback is inevitable because the wide channel is necessary to obtain the crescent-shaped active layer in this prior art.